Magnetic resonance imaging (MRI) is used to generate medical diagnostic images by measuring faint radio frequency signals (magnetic resonance) emitted from atomic nuclei of tissue (for example, hydrogen atoms in water molecules) in the presence of a strong magnetic field after radio frequency stimulation.
The radio frequency stimulation may be applied, and the resulting resonance signal detected with a xe2x80x9clocal coilxe2x80x9d having one or more xe2x80x9cloopsxe2x80x9d serving as antennas. The loops of the local coil are tuned to a narrow band (e.g., 64 megahertz for a one and one half Tesla magnetic field strength) and adapted to be placed near or on the patient to decrease the effects of external electrical noise on the detected signal. The detected signal may be conducted through one or more signal cables to the MRI system for processing. Signals from multiple loops may be combined prior to being processed by the MRI system, for example, as in a quadrature-type coil where perpendicular loops are combined with a ninety degree phase shift, or the signals may be conducted independently to the MRI system to provide for so-called phased array operation.
The low strength of the detected resonance signals requires that the signal cables be shielded from external radio frequency interference. This interference may come from the external hospital environment, the gradient coils of the MRI machine, or from other loops of the local coil itself. Particularly, for phased array operation, it is important that the signals of each loop of the local coils be kept isolated from the signals from the other loops.
Cross-coupling of the signals between adjacent loops can be minimized by careful routing of the signal cables along regions of low electrical field strength about the coil. Unfortunately, the regions of low electrical field strength may shift during use of the coil or be difficult to determine, or may be in areas where the routing of cables is undesirable, interfering with patient access or the like, or unnecessarily increasing cable length and contributing to signal loss.
The present invention provides a method of routing signal cables along low electrical field strength areas of a local coil in a predictable and reliable manner. The invention introduces a conductive ring into proximity with the coil loops and the ring is grounded thus creating a low electrical field strength area. The signal cables are attached to the conductor of the ring to run along the ring to a point where they exit together as a unified cable. The ring, by defining an area of low electromagnetic field, reduces interference on the cables.
One object of the invention is to provide for a method of extracting signals from multiple loop coils, such as phased array coils, while minimizing the electric interference among the signals.
It is another object of the invention to allow the convenient routing of the cables so as not to interfere with use of the coil. The ring allows the routing path to be flexibly selected.
It is yet another object of the invention to allow multiple cables from each coil to be collected into a single cable without unduly increasing the electrical interference received by those cables that must have a longer path length.
It is a further object of the invention to equalize the potential in the vicinity of the ring.
The signal cables may be shielded cables having an outer shield surrounding an inner conductor and the outer shields may be electrically connected to the conductive ring. Thus, it is another object of the invention to provide a simple method of grounding the ring.
The conductive ring may be substantially parallel and co-planar with the loops, for example, when the loops define a plane or surface, or the conductive ring may be substantially perpendicular to the loops, or when the loops define the surface of a cylinder with the conductive ring forming a base of the cylinder.
Thus, it is another object of the invention to provide a coil routing system adaptable to common coil designs.
The loops may be symmetric with the respect to the conductive ring and therefore another object of the invention may be to provide a conductive ring orientation that reduces net current flow in the ring, therefore reducing current flow in the shields of the cables attached to the ring.
The foregoing objects and advantages may not apply to all embodiments of the inventions as claimed and are not intended to define the scope of the invention, for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.